HORTICULTURERoots: The Hidden Half of Growth
Mark Longstroth
Southwest Michigan District Extension Educator for Fruit
HORTICULTURERoots: The Hidden Half of Growth
Mark Longstroth
Southwest Michigan District Extension Educator for Fruit
Winter has come and the trees, vines and bushes that bear our Michigan fruit crops have lost their leaves and gone dormant for the winter. Fruit growers would be wrong to think that not much is happening in their orchards and vineyards. The above ground portions of our plants are dormant and the buds by some undetermined mechanism are keeping track of chilling hours. We may think that the whole plant is dormant but this is hardly the case. In the ground, the roots are active and growing. Unlike the above ground shoots, which are dormant and will not grow until completing their chilling requirement, the only thing holding back root growth is soil temperature. The roots are capable of growth at any time of the year. All the roots need are conditions conducive to growth and the lack of competition from other plant organs such as the shoots and fruit.
All of us know that our plants have roots. But we just don't think about them. It's hard to image things we can not see, touch or feel. Most of us, when we think of the roots imagine a subterranean mirror image of the trunk. We often wonder how far from the tree do the roots go? How deep are they? We busily take care of the plant above ground but we seldom think about the roots. With this essay, I hope to give you a little insight into what is happening under your feet all year long.
We know the roots provide support for the tree. We are reminded of this when trees fall over. We also know that the root system absorbs and supplies water and nutrients to the tree. We are reminded that the tree needs roots when new plants leaf out and then just sit there not growing for the rest of the growing season. We often wonder what could we do to increase growth. Then the next year, the tree takes off, growing like a rocket. This situation often occurs when trees are planted late or the soil conditions are not favorable for root growth. The young plant is unable to produce enough of a root system to supply the needs of the plant. When the demands of the leaves overtake the ability of the roots to supply water, growth stops.
In woody plants, there are two kinds of roots; woody roots and white roots. These two kinds are very different. The woody roots are in essence the underground trunk of the tree. These are the roots that we all think of when we think to the root system of a perennial plant. There are also new white roots that are the result of active growth. These young roots are an important site of mineral absorption. White roots do not hang around long. The live span of a young root is measured in days and weeks. As the white portion of the root matures, it turns brown as the outer layers turn brown and die. Nine times out of ten the remaining small brown root also dies. Occasionally a brown root lives. The root thickens and becomes part of the woody root system. Root survival is probably due to whether there is still good growth in the root tips supported by that root.
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The picture at left is part of a photograph I took long ago in the
rhizotron at the University of Michigan's Forest Research Station near Pellston Michigan.
This picture shows white (upper left) and brown roots (right). The roots that are changing from brown to white on the right side of the picture are in the process of either dying or becoming woody permanent roots. The bright white root on the left is a young actively growing root. The rhizotron is a root observatory with windows mounted in the soil to observe root growth. The wires embedded in the glass from a one centimeter grid allowing measurement and other observations. |
Root growth is similar to shoot growth but there are major differences. At the tip of an actively growing root is the root cap. The root cap is a loose assemblage of cells and a slimly substance called mucilage that protects and lubricates the root tip as it forces its way through the soil. Behind the root cap is the zone of active cell division where new cells are created. Immediately behind is the zone of cell elongation. This is where the new cells begin growth. Plant cells grow by pumping themselves full of water. They do this by actively moving salt ions, usually potassium, into the cell. They also move sugars into the cell. The name of the game is to be saltier and sweeter that the water around you. This causes water to move into the cell. The cells do not actively take up water. The water flows into the cells because there are more salts and sugars inside the cells than outside. This is the way all plant cells grow and the way roots absorb water from the soil. As the cells absorb water they increase in length and force the root tip deeper into the soil.
Behind the zone of elongation is the zone of maturation where the different types of cells develop. The xylem, long cells that conduct water out of the roots and up the tree, form in the middle of the root. On the outside of the root is the cortex that absorbs nutrients from the soil. In-between the xylem and the cortex are two very important layers of cells. The outer layer is the endodermis. This layer of cells forms a thick waxy layer between all its cells called the casparian strip. This thick barrier forces water and nutrients moving into the xylem to go through the endodermis cells. This barrier prevents leakage out of the root. Inside the endodermis is the pericycle. This layer still has the capacity for new growth by renewed cell division. This is where new roots originate and where thickening will occur if the root survives and becomes woody. As the root ages the cortex dies and the endodermis is exposed to the soil. This is when the white root turns brown and soon after that the root usually dies. If the root lives, new roots from the pericycle may burst out through the old covering or the root may thicken and become woody.
There is a pattern and timing to root growth. The roots grow when the shoots are not growing. Competition with other parts of the plant is the main determining factor to root growth. In fall, after harvest, sugars made in the leaves was transferred to the shoots, trunk and roots for storage and use during the next growing season. A large portion of these storage compounds are stored in the roots. The roots use this abundance of energy for growth and root growth is active in the late summer and fall. Root growth begins to taper off as the soil cools in the fall, but the roots are always capable of growth. As the soil becomes cold in the winter, root growth ceases. This is not because the roots are dormant as the buds are in the top of the tree, but because the soil is too cool to support growth. The crown of the tree, where the root system joins the trunk of the tree is the transition zone between the dormant top and the active roots. This transition zone, which often contains a graft union, is very susceptible to winter cold injury. Conditions that favor late root growth can predispose your plants to cold injury in the winter, particularly if the ground is not covered by snow.
Root growth increases in the Spring. This is not because the roots have become active but because the soil is now warm enough to support faster growth. Root growth increases as the soil warms. Rapid root growth continues through bud break and only begins to slow as the shoots begin their growth. During the period of rapid shoot growth in the early spring new root growth virtually stops and all the trees' reserves are used in above ground growth. When shoot growth ceases in early summer, root growth begins again. The rate of root growth increases through the summer reaching a peak in early fall and then declines as the soil cools. So, two of the main restraints on root growth are soil temperature and competition with shoots and fruit.
The other factor determining root growth is soil moisture. Roots grow where the conditions for growth are favorable. The old adage that roots grow to water is WRONG. Roots grow where the water is. Roots in dry soil show very little growth. In the spring, when the soil is moist and the surface is warm, roots near the surface show the most growth. As the soil dries out, roots near the surface die and new roots grow deeper in the soil. As the season progresses, the depth of active root growth moves progressively deeper and deeper in the soil. With each rain or irrigation new roots appear near the surface. These new roots absorb water and nutrients and then die.
Lets go back to the analogy of the upside down tree to represent the root system. We can imagine that instead of one trunk that there are several and the trunk and branches of this root system are composed of woody roots. Now imagine that the leaves on this tree are not green and flat like an apple or a grape leaf but long and thin like the needles of a pine and white not green. Keep in mind that most of these leaves (white roots) will fall off, many a week or two after they form, and few become permanent woody stems. Unlike the leaves on a pine tree, white roots can change into a woody roots (stems) and grow more leaves (roots).
I find that thinking of white roots as leaves helps me to put the process of root growth in a visual framework that I can understand. The white roots really are like leaves. They are temporary structures that absorb water and nutrients. Leaves are temporary structures that absorb carbon dioxide and make sugar providing the energy for plant growth and fruiting. When their job is done, both roots and leaves are discarded by the plant.
As the growth of our imaginary (upside down) root tree begins in the Spring, most of the roots sprout out close to the ground. As the season progresses, more roots sprout deeper and deeper in the soil. Instead of hanging on to the trunk all season, most of the roots fall off after two weeks. Rapidly the white rooted region of the tree moves downward in the soil, and every time it rains there is a new flush of growth near the surface of the soil from the older root system. Towards the end of the year, fewer new roots grow deep in the soil. Most of the new growth occurs close to the soil surface where the ground is warm, moist and has the most nutrients. Over the years a frame work builds, with many branches close to the soil surface and several large trunks. Most of these large trunks lie close to the surface. Few roots are found deep in the soil. Forty to sixty percent of the root system of perennial plants is in the uppermost layer of the soil. On the large woody roots there are branches where the roots have found water and nutrients over the years.
If you now take this root system and place it in the soil with a fruit
tree above we can visualize the annual cycle of root and shoot growth. In the early
Spring, many new roots grow close to the soil surface and remove water from the surface
soil layers. As the leaves begin to appear on the tree, roots appear lower and lower in
the soil. While the leaves above ground last until the end of the growing season, there
are ten or twenty overlapping generations of new roots in the soil. You can almost imagine
a time lapse movie of tree growth. Above the ground we see leaves form and shoots grow
begin. Below ground we see a zone of white moving downward in the soil, broadening and
narrowing as it moves downward. As active shoot growth begins, root growth slows and the
white zone fades. All through the soil we see brief flashes of white as roots grow and
die. As shoot growth ceases, we see increased activity in the soil as new roots form close
to the surface and older, deeper roots die.
Almost half the sugars manufactured in the leaves are used and lost through root growth,
which we normally do not think about because we do not see it. As I sit here and write
this on a cold winter day, I can look outside where the trees are standing with no leaves
on them. I know that in the soil roots are growing, getting ready to supply that explosive
burst of green, leafy new growth that comes every spring.
This is a revised version of an article that first appeared in The Great Lakes Fruit Growers News, Vol. 33, No 11, Nov. 1994, as: Root Development is the Hidden Half of Tree Growth.
